In the automotive industry, in order to reduce the CO2 emission from the standpoint of global environmental conservation, improving the fuel consumption of automobiles by reducing the weight of automobile bodies while maintaining the strength thereof has been always an important issue. In order to reduce the weight of automobile bodies while maintaining the strength thereof, it is effective to reduce the thickness of a steel sheet used as a material of automotive parts by increasing the strength of the steel sheet.
Meanwhile, many of automotive parts composed of a steel sheet material are formed by press forming, burring, or the like. Therefore, it is desired that a high-strength steel sheet used as a material of automotive parts have excellent formability, namely, ductility and stretch flangeability in addition to a desired strength.
Furthermore, in a material of automotive parts, one of properties on which the greatest importance should be placed is crashworthiness. At the time of the crash of an automobile, each position of the automobile composed of a steel sheet is subjected to a strain rate of no less than about 103/s. Accordingly, it is necessary to ensure crash safety of automobiles by applying, to automotive parts such as a pillar, a member, and a bumper, a high-strength steel sheet having crashworthiness sufficient to ensure security of an occupant in case of crash during driving of the automobiles, that is, a high-strength steel sheet having crashworthiness in which excellent crash energy absorption is exhibited even in the case where the steel sheet is subjected to such a high strain rate at the time of the crash.
For the above reason, particularly in the automotive industry, there has been a strong desire to develop a high-strength steel sheet having not only strength but also formability such as ductility and stretch flangeability, and further crashworthiness. Thus, many research and development have been conducted to date, and various technologies have been proposed.
For example, Patent Literature 1 has proposed a technology related to a ferrite-martensite dual-phase (DP) steel sheet, in which a yield stress at a strain rate of 103/s is increased by adjusting the average grain diameter and the volume ratio of each of ferrite and martensite to improve crashworthiness. However, the reason why the DP steel sheet, which originally has a low yield strength, exhibits high energy absorption is that a relatively large work strain is introduced by press forming or the like, and strain aging occurs in a subsequent paint-baking step, thereby significantly increasing the yield stress. Therefore, at a position (in a part) that is subjected to a light degree of forming such as bending, since the work strain to be introduced is small, a significant effect of increasing the yield stress cannot be expected after the paint-baking step. Thus, there is a problem in that sufficient crash energy absorption is not necessarily exhibited.
In addition, the DP steel sheet is characterized by exhibiting excellent crash energy absorption in a high strain range of 10% to 30%, but the DP steel sheet does not exhibit sufficient crash energy absorption in a low strain range. Thus, the DP steel sheet is suitable for use in a position (part) that absorbs crash energy by a certain degree of deformation, such as a position (part) that is subjected to frontal crash. However, crashworthiness of the DP steel sheet is insufficient when the DP steel sheet is applied to a position (part) that requires high crash energy absorption in a small strain range without significant deformation from the standpoint of protecting an occupant, such as a position (part) that is subjected to side crash.
Patent Literature 2 has proposed a technology related to a transformation induced plasticity (TRIP) steel sheet that utilizes transformation induced plasticity of retained austenite, in which the amount of bake hardening is increased by adjusting the amount of bainite to improve crash energy absorption. However, as in the DP steel sheet, the TRIP steel sheet also has a problem in that the TRIP steel sheet does not necessarily exhibit sufficient crash energy absorption at a position (in a part) that is subjected to a light degree of forming such as bending, and is not suitable for use in a position (part) that requires high crash energy absorption in a small strain range.
In relation to the above related art, Patent Literature 3 has proposed a technology related to a cold-rolled steel sheet having a microstructure mainly composed of ferrite, in which crashworthiness of the steel sheet is improved by adjusting the volume ratio and the average crystal grain diameter of a low-temperature transformed phase composed of at least one of martensite, bainite, and retained austenite, and the average distance between the low-temperature transformed phases.
However, in the technology proposed in Patent Literature 3, steel sheet properties other than crashworthiness are insufficient. In this technology, since the steel sheet has a microstructure mainly compose of ferrite, the tensile strength (TS) of the steel sheet is less than 1,200 MPa, and thus a satisfactory strength is not obtained. In addition, in this technology, stretch flangeability of the steel sheet is not examined, and thus this steel sheet may not have satisfactory formability.
Since automotive parts are often used in a severe corrosion environment, recently, a high-strength galvanized steel sheet, which has a high strength and excellent corrosion resistance, has been widely used as a material of automotive parts. Furthermore, nowadays, a further increase in the strength has been promoted in materials of automotive parts, and the application of a steel sheet having a tensile strength of 1,200 MPa or more has been studied.
In response to the above requirement, for example, Patent Literature 4 has proposed a technology related to a steel sheet having a microstructure mainly composed of tempered martensite, in which not only an increase in the strength but also an improvement in ductility and stretch flangeability is achieved by adjusting the area ratios of martensite, bainite, and retained austenite. According to this technology, it is possible to obtain a galvanized steel sheet having a high strength, i.e., tensile strength (TS): 1,200 MPa or more and excellent workability.
However, in the technology proposed in Patent Literature 4, crashworthiness of the steel sheet is not examined. Therefore, according to this technology, although a galvanized steel sheet having a high strength and excellent formability is obtained, crashworthiness thereof may not be sufficient. Thus, in particular, it may be possible to further improve crash energy absorption in a small strain range.